Attention has been attracted to high-output laser light sources with an output exceeding several W as light sources used for processing application or used in laser displays. Semiconductor lasers using gallium arsenide, gallium nitride and the like have been developed for the generation of light in a red or blue region, and higher outputs are also being studied. However, it is still difficult to directly generate light in a green region from a semiconductor laser. Thus, a general method is such that infrared light emitted from a solid-state laser such as a YAG laser or a fiber laser using a fiber doped with a rare earth such as Yb or Nd is incident as a fundamental wave on a nonlinear optical crystal to obtain green light as a second harmonic wave by a wavelength conversion.
Particularly, a quasi phase matching wavelength conversion element formed using a polarization reversal technology is so constructed as to enable the generation of high-output short-wavelength light in a nonlinear optical crystal composed of LiNbO3 (hereinafter, “LN”) or LiTaO3 (hereinafter, “LT”). In the LN or LT nonlinear optical crystal composed of LN or LT, optical damage at the time of generating short-wavelength light has been a problem. This is a phenomenon of changing a refractive index by an electric field distribution formed in the crystal by the short-wavelength light and can be reduced by adding a necessary amount of Mg, In, Zn, Sc or the like in the LN or LT nonlinear optical crystal. On the other hand, it is known that optical damage can be reduced to a certain degree by keeping crystals at a high temperature of 100° C. or higher in non-doped crystals not added with these additives.
In other words, the LN or LT nonlinear optical crystal has been known to be able to reduce an output variation caused by optical damage by using a non-doped crystal at a high temperature or adding an additive. For example, non-patent literatures 1 and 2 disclose that optical damage can be suppressed by adding MgO in a molar concentration of 5.0 mol % or more.
On the other hand, even in crystals with suppressed optical damage, output instability for short-wavelength light with an output exceeding several W and a phenomenon of causing a crystal destruction have been found. For example, as disclosed in patent literature 1, green light absorption is induced by ultraviolet light (third harmonic wave) generated as a sum frequency of infrared light as a fundamental wave and converted green light (second harmonic wave) and the crystal destruction occurs due to green light absorption at the time of a high-output wavelength conversion. In this case, a wavelength conversion exceeding several W becomes difficult.
In order to provide a light source for generating green light with an output exceeding several W necessary for medical, machining or laser display use, a nonlinear optical crystal which has less absorption of green light induced by ultraviolet light and does not cause optical damage is strongly required.
In the conventional construction, output instability caused by optical damage is solved in the LN or LT nonlinear optical crystal including the additive such as Mg, In, Zn or Sc. However, in the generation of short-wavelength light with an output exceeding several W, output instability and phenomena such as crystal destruction resulting from a thermal lens effect by light absorption are not solved and there has been a problem of being difficult to obtain a higher output from a nonlinear optical crystal including an additive.
Specifically, in the case of obtaining a harmonic wave of several W using a wavelength conversion element composed of MgLN (LN nonlinear optical crystal added with Mg), ultraviolet light (third harmonic wave) as a sum frequency of infrared light as a fundamental wave and converted green light (second harmonic wave) is generated due to a large nonlinear optical constant also in the case of deviation from a phase matching condition. This generated ultraviolet light induces the green light absorption to create a thermal lens. This has caused a problem of inducing the beam deterioration of the green light and a problem of inducing a reduction in conversion efficiency at the time of a high output and the thermal destruction of the crystal by generated heat.
Although it depends on the element, crystal destruction starts upon generating an output exceeding 2.5 W in the case of generating green light. In the case of generating blue light having a shorter wavelength than green light, it is known that a threshold value of the crystal destruction is reduced and the crystal destruction starts when an average output of continuous light exceeds an output of 2 W. In the case of pulse oscillation with a high peak value, the crystal destruction occurs when an average output exceeds 0.5 W.
There has been a method for suppressing optical damage by increasing a crystal temperature in a non-doped LN or LT nonlinear optical crystal having a conventional structure. However, a reduction of optical damage by an operation at a high temperature requires a high temperature of 100° C. or higher and the realization of a high output characteristic requires a high temperature of 140° C. or higher. Further, it is difficult to completely reduce optical damage even in the case of an operation at a high temperature. Particularly, in the generation of short-wavelength light, there has been a problem that an output becomes unstable. In the operation at a high temperature of 140° C. or higher, there have been a problem of being difficult to maintain temperature uniformity, a problem of increasing power consumption and other problems. Further, since a temperature tolerance of the wavelength conversion element having a periodical polarization reversal structure becomes narrower as temperature rises, there have been a problem of necessitating a precise temperature control at high temperatures and a problem of being difficult to stabilize an output.
Patent Literature 1:
Japanese Unexamined Patent Publication No. 2006-308731
Non-Patent Literature 1:
D. A. Bryan, Robert Gerson, H. E. Tomaschke, “Increased Optical Damage Resistance in Lithium Niobate”, Applied Physics letters, 44(9), 1984, pp. 847-849
Non-Patent Literature 2:
D. H. Jundt, G. A. Magel, M. M. Fejer, R. L. Byer, “Periodically Poled LiNbO3 for High-Efficiency Second-Harmonic Generation”, Applied Physics letters, 59(21), 1991, pp. 2657-2659